1. FIELD OF THE INVENTION
The present invention relates to the cooling of process gases and, particularly, to the transfer of waste heat from a gas to steam generator feed water. More specifically, this invention is directed to improvements in and to heat exchangers and especially to heat exchangers of the type wherein a cooling shield is provided about a generally linear section of a conduit through which a hot gas flows. Accordingly, the general objects of the present invention are to provide novel an improved methods and apparatus of such character.
2. DESCRIPTION OF THE PRIOR ART
Heat exchangers of the type wherein a centrally arranged pipe is surrounded, at least over a significant portion of its length, by a cooling shield or jacket are well known in the art. Such heat exchangers are often used for the cooling of process gases, particularly fission gases resulting from the thermal fission of gaseous and liquid hydrocarbons. These heat exchangers are designed as "one-pipe" devices which connect to respective individual slit tube outlets of the fission oven, the heat exchangers having an internal cross-section which corresponds to that of the fission oven outlet at least at the gas inlet side of the exchanger. For an example of such prior art "one-pipe" heat exchangers, reference may be had to German Patent No. 19 29 479.
Prior art heat exchangers of the "one-pipe" type are typically constructed such that the interior pipe, through which the hot gas flows, is enclosed by a shield pipe. The shield pipe defines an annular chamber through which steam generator feed water, generally under high pressure, will flow. This steam generator feed located between the two pipes thus functions as a cooling medium. The interior pipe which conducts the gas is subjected to the high steam generator pressure from the outside. Since the interior pipe, i.e. the pipe conducting the process gas, must be designed to withstand the high outside steam generating pressure, this pipe must be characterized by a relatively thick wall. Further, given the prevailing heating surface loads in the range of 100-500 kW/m.sup.2, the wall of the interior pipe is also subjected to high thermal tension and to a high median temperature. This combination of high temperature, high thermal tension and high applied exterior pressure results in potential damage and possible premature failure. Further, because of the mechanical and thermal stresses, it has not been possible to select and utilize the optimum materials, taking into account the process gas conditions and steam generator operation, for both the interior pipe and the shield pipe.